Rhesus monkeys (10) and pigeons (24) will be trained and tested in 4 cohorts to determine the mechanisms of visual list memory and how list memory changes over the course of retention delay. The different cohorts will provide experiential and task comparisons to broaden the findings from these experiments. Experiments with lists of 1-4 pictures (travel slides) and retention delays of 0-30 seconds will test how some list items inhibit memory of other list items and how this inhibition changes with retention delay. Experiments manipulating intervals between list items will test how inhibition among list items changes with item separation and with retention delay. Experiments with items for which memory performance has been degraded (through repetition) will test how the inhibitory effect of these difficult-to-remember items is altered while at the same time maintaining the same number of items and the same interval relationships. Other experiments will test how these subjects decide whether a test item was in the current memory list by creating conflict (i.e., proactive interference). They will be tested for how far back in time previous list items can interfere (or enhance) with their memory performance, thereby determining whether they use familiarity exclusively or can be trained to restrict their decisions more to the current memory list. As a whole, the experiments are designed to produce converging results and determine how different memorial processes interact and change with retention delay to influence which list items are best remembered (i.e., the serial position function). The research of this proposal should be significant in its contributions to a basic understanding of memorial processes in two nonverbal animal species, one relatively closely related to humans (rhesus monkeys) and one distantly related (pigeons), and will provide behavioral evidence critical to related research (e.g., neurophysiological) on how memory works.